They are called Identicons. The Gravatar identicon image is based off of an MD5 hash of your email address. Here is an identicon implmentation if you are using .NET. There are other implementations listed in the Wikipedia article as well.

An Identicon is a visual
representation of a hash value,
usually of the IP address, serving to
identify a user of a computer system.
The original Identicon is a 9-block
graphic, which has been extended to
other graphic forms by third parties
some of whom have used MD5 instead of
the IP address as the identifier. In
summary, an Identicon is a privacy
protecting derivative of each user's
IP address built into a 9-block image
and displayed next the user's name. A
visual representation is thought to be
easier to compare than one which uses
only numbers and more importantly, it
maintains the person's privacy. The
Identicon graphic is unique since it's
based on the users IP, but it is not
possible to recover the IP by looking
at the Identicon.

Link on there is dead, there's an (I think) older version on CodePlex: identicon.codeplex.com. There were some minor (Dispose) issues with it, so I've just uploaded a patch for this (and a little performance improvement).
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WoutSep 23 '12 at 18:54